+ mm-memcg-disable-threshold-event-handlers-on-preempt_rt.patch added to -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: mm/memcg: disable threshold event handlers on PREEMPT_RT
has been added to the -mm tree.  Its filename is
     mm-memcg-disable-threshold-event-handlers-on-preempt_rt.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-memcg-disable-threshold-event-handlers-on-preempt_rt.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-memcg-disable-threshold-event-handlers-on-preempt_rt.patch

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------------------------------------------------------
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: mm/memcg: disable threshold event handlers on PREEMPT_RT

During the integration of PREEMPT_RT support, the code flow around
memcg_check_events() resulted in `twisted code'.  Moving the code around
and avoiding then would then lead to an additional local-irq-save section
within memcg_check_events().  While looking better, it adds a
local-irq-save section to code flow which is usually within an
local-irq-off block on non-PREEMPT_RT configurations.

The threshold event handler is a deprecated memcg v1 feature.  Instead of
trying to get it to work under PREEMPT_RT just disable it.  There should
be no users on PREEMPT_RT.  From that perspective it makes even less sense
to get it to work under PREEMPT_RT while having zero users.

Make memory.soft_limit_in_bytes and cgroup.event_control return
-EOPNOTSUPP on PREEMPT_RT.  Make an empty memcg_check_events() and
memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT. 
Document that the two knobs are disabled on PREEMPT_RT.

Link: https://lkml.kernel.org/r/20220221182540.380526-3-bigeasy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Suggested-by: Michal Hocko <mhocko@kernel.org>
Suggested-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: kernel test robot <oliver.sang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 Documentation/admin-guide/cgroup-v1/memory.rst |    2 ++
 mm/memcontrol.c                                |   14 ++++++++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

--- a/Documentation/admin-guide/cgroup-v1/memory.rst~mm-memcg-disable-threshold-event-handlers-on-preempt_rt
+++ a/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -64,6 +64,7 @@ Brief summary of control files.
 				     threads
  cgroup.procs			     show list of processes
  cgroup.event_control		     an interface for event_fd()
+				     This knob is not available on CONFIG_PREEMPT_RT systems.
  memory.usage_in_bytes		     show current usage for memory
 				     (See 5.5 for details)
  memory.memsw.usage_in_bytes	     show current usage for memory+Swap
@@ -75,6 +76,7 @@ Brief summary of control files.
  memory.max_usage_in_bytes	     show max memory usage recorded
  memory.memsw.max_usage_in_bytes     show max memory+Swap usage recorded
  memory.soft_limit_in_bytes	     set/show soft limit of memory usage
+				     This knob is not available on CONFIG_PREEMPT_RT systems.
  memory.stat			     show various statistics
  memory.use_hierarchy		     set/show hierarchical account enabled
                                      This knob is deprecated and shouldn't be
--- a/mm/memcontrol.c~mm-memcg-disable-threshold-event-handlers-on-preempt_rt
+++ a/mm/memcontrol.c
@@ -858,6 +858,9 @@ static bool mem_cgroup_event_ratelimit(s
  */
 static void memcg_check_events(struct mem_cgroup *memcg, int nid)
 {
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		return;
+
 	/* threshold event is triggered in finer grain than soft limit */
 	if (unlikely(mem_cgroup_event_ratelimit(memcg,
 						MEM_CGROUP_TARGET_THRESH))) {
@@ -3731,8 +3734,12 @@ static ssize_t mem_cgroup_write(struct k
 		}
 		break;
 	case RES_SOFT_LIMIT:
-		memcg->soft_limit = nr_pages;
-		ret = 0;
+		if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+			ret = -EOPNOTSUPP;
+		} else {
+			memcg->soft_limit = nr_pages;
+			ret = 0;
+		}
 		break;
 	}
 	return ret ?: nbytes;
@@ -4708,6 +4715,9 @@ static ssize_t memcg_write_event_control
 	char *endp;
 	int ret;
 
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		return -EOPNOTSUPP;
+
 	buf = strstrip(buf);
 
 	efd = simple_strtoul(buf, &endp, 10);
_

Patches currently in -mm which might be from bigeasy@linutronix.de are

mm-memcg-disable-threshold-event-handlers-on-preempt_rt.patch
mm-memcg-protect-per-cpu-counter-by-disabling-preemption-on-preempt_rt-where-needed.patch
mm-memcg-protect-memcg_stock-with-a-local_lock_t.patch
mm-memcg-disable-migration-instead-of-preemption-in-drain_all_stock.patch